1. Field of the Invention
The present invention relates to a downhole tool for oil and/or gas production. More specifically, the invention is a well stimulation tool having an expandable seat for use with a tubing string disposed in a hydrocarbon well.
2. Description of the Related Art
In hydrocarbon wells, fracturing (or “fracing”) is a technique used by well operators to create and/or extend a fracture from the wellbore deeper into the surrounding formation, thus increasing the surface area for formation fluids to flow into the well. Fracing is typically accomplished by either injecting fluids into the formation at high pressure (hydraulic fracturing) or injecting fluids laced with round granular material (proppant fracturing) into the formation.
Fracing multiple-stage production wells requires selective actuation of downhole tools, such as fracing valves, to control fluid flow from the tubing string to the formation. For example, U.S. Published Application No. 2008/0302538, entitled Cemented Open Hole Selective Fracing System and which is incorporated by reference herein, describes one system for selectively actuating a fracing sleeve that incorporates a shifting tool. The tool is run into the tubing string and engages with a profile within the interior of the valve. An inner sleeve may then be moved to an open position to allow fracing or to a closed position to prevent fluid flow to or from the formation.
That same application describes a system using multiple ball-and-seat tools, each having a differently-sized ball seat and corresponding ball. Ball-and-seat systems are simpler actuating mechanisms than shifting tools and do not require running such tools thousands of feet into the tubing string. Most ball-and-seat systems allow a one-quarter inch difference between sleeves and the inner diameters of the seats of the valves within the string. For example, in a 4.5-inch liner, it would be common to drop balls from 1.25-inches in diameter to 3.5-inches in diameters in one-quarter inch or one-eighth inch increments, with the smallest ball seat positioned in the last valve in the tubing string. This, however, limits the number of valves that can be used in a given tubing string because each ball would only be able to actuate a single valve, the size of the liner only provides for a set number of valves with differently-sized ball seats. In other words, because a ball must be larger than the ball seat of the valve to be actuated and smaller than the ball seats of all upwell valve, each ball can only actuate one tool.